1. Field of the Invention
This invention relates to a method of accelerating the breakdown of phenolic resin bonded cores and to additive compositions for its acceleration.
2. Description of the Prior Art
Cores bonded with phenolic resin have been widely used in foundries of metals ranging from steel to aluminum. However, in some cases such as aluminum casting, this type of core has the difficult problem that it fails in breakdown capability This is due to the fact that the melting temperature of aluminum is too low to promote the thermal decomposition of phenolic resin during its casting. In addition, there are other causes which are characteristic of aluminum casting and which worsen the problem.
Phenolic resin is heat resistant and undergoes thermal decomposition only when it is heated above 500.degree. C. Hence, cores bonded with the resin fail in breakdown capability in aluminum castings whose melting temperature is too low to heat them up to the critical temperature. In most aluminum casting methods, pouring is at around 700.degree. C. and the temperature to which cores are heated is estimated to be below 500.degree. C. For shaking out cores completely, it is often necessary to reheat castings in a furnace above 500.degree. C. for several hours. Naturally, this results not only in excessive man-hours but also in high fuel costs.
To improve the breakdown property of phenolic resin bonded cores, various solutions have been proposed. For example, various derivatives of phenolic resin having a lower thermal decomposition temperature are used for core making. However, cores thus made bring about another undesirable result in aluminum castings. That is, since thermal decomposition of resin is always accompanied by gas evolution, it tends to cause blow holes in castings. In particular, there is a tendency that the lower the thermal decomposition temperature of the resin is, the more marked is the production of blow holes are caused.